Conventional iris devices are known in the art and are commonly used in optical imaging systems. A single iris is the simplest embodiment of an iris device. The iris or aperture stop is an important element in most optical systems. The iris of an optical system limits the amount of light passing through said optical system and reaching a sensor. If the iris is not embodied as a separate element, the optical elements of the optical system itself may constitute the iris, e.g. by the diameter of a lens.
However, as monochromatic aberrations of the optical system increase with the possible distance of a light beam, which is a common model for the description of optical systems, to the optical axis of the optical system, it is often desired to limit the light beams passing through the optical system to paraxial light beams, i.e. light beams in the proximity of the optical axis.
In the art, commonly iris diaphragms are applied for this purpose. Such iris diaphragms allow to variably determine the amount of light which is passing through the optical system.
The iris or aperture stop further effects the depth of field (DOF) of the optical system. The DOF, also called depth of sharpness or depth of focus, defines the range of distances which can be focused by an optical imaging system.
The DOF shows an inverse functional dependence on the diameter of the aperture, wherein the diameter of the aperture corresponds to twice the distance of a light beam farthest away from the optical axis. Those light beams with the maximum possible distance away from the optical axis may be referred to as marginal rays.
In other words, the larger the diameter of the aperture, the shallower is the DOF. A decreasing diameter of the aperture, on the contrary, increases the DOF. Consequently, in irises of the art, the amount of light passing through an optical system, in particular an optical imaging system, may only be increased at the expense of a shallower DOF.
Accordingly, an increased DOF may only be obtained at the expense of a decreased amount of light transmitted through the optical system.
Therefore, a conventional iris unavoidably results in a trade-off between the DOF and the amount of light transmitted through the optical system, i.e. in particular the amount of light incident on the sensor.
Surgical microscopes, as an exemplary field of application, offer different imaging modes which may have different optical iris settings. Specifically, in color reflectance mode, illumination of a sample may be sufficiently intense, and conveniently a conventional iris is used to set a small diameter which increases the DOF, thus allowing better focus of uneven surfaces.
In a fluorescence mode of the exemplary surgical microscope, a sample is excited by illumination with radiation having an excitation wavelength, wherein the sample emits fluorescent light upon its excitation. The fluorescent light is commonly red-shifted, i.e. the fluorescence wavelength is longer than the excitation wavelength.
As the number of photons of the fluorescence emitted per exciting photon, i.e. the quantum yield, is commonly well below 10%, the iris should be open to collect as many of the fluorescence photons as possible.
The simultaneous inspection of a sample in color reflectance mode and fluorescence mode therefore requires a compromise, since a conventional iris does not allow simultaneous application of different aperture sizes. Either the DOF is increased at the expense of the fluorescence sensitivity or the fluorescence sensitivity is increased at the expense of a decreased DOF.
An iris of the art transmits or rejects light in the same way for all wavelengths, i.e. it is not wavelength-selective and a shows a spectrally flat transmission property.